This invention is in the field of vascular grafts made of synthetic fibers that are used in a variety of vascular surgical procedures. Particular interest herein is to grafts used to replace occluded portions of arteriosclerotic vessels or used to form new blood pathways in vascular reconstruction procedures, as generally disclosed in U.S. Pat. Nos. 4,517,687, 4,047,252, 4,530,113, and 3,878,565.
There are basically three kinds of grafts on the market, namely extruded, knitted and woven. Extruded or non-woven grafts are generally strong and non-porous which precludes subsequent hemorrhaging, but have numerous undesirable characteristics. The total lack of porosity prevents subsequent tissue ingrowth which has been considered highly desirable. Also these grafts are relatively stiff and nonconforming and thus difficult to handle and implant.
Knitted grafts have numerous advantages over the extruded ones, namely high porosity, flexibility, softness and a velour type surface. Accordingly, they conform easily which reduces surgeon's and patient's time in the operating room, and the porosity and velour surface allow considerable endothelialization (tissue ingrowth). The principal disadvantage of knitted grafts is that the porosity is so great, hemorrhaging will occur unless the graft is preclotted, this being a separate step requiring about fifteen minutes for immersion of the graft in a quantity of about 100-150 cc of blood of the patient, after which the graft is allowed to stand for clotting to occur. Preclotting substantially prevents hemorrhaging, while tissue ingrowth can still proceed; however sometimes preclotting is not permissible as where the patient has been anti-coagulated or has bleeding diathesis. In these cases a knit graft cannot be used. Final negative considerations about knitted grafts are the recent belief that the high velour interior surface inherent in knit fabric may simply act to collect dead tissue and disrupt blood flow, that tissue ingrowth into the high velour exterior surface may be less significant than previously predicted, and that knitted grafts expand and stretch more than other types.
Woven grafts have certain advantages over both extruded and knitted grafts in that porosity is lower, so that preclotting is not required and the surface are more uniform for smoother blood flow. A disadvantage of woven grafts is that they are relatively stiffer and less conforming than knit grafts and thus more difficult and time consuming for the surgeon to use.
A prospective and not currently available graft is a knitted tube coated in manufacture with a substance such as collagen to prevent initial bleeding following implantation. This collagen coating obviates the necessity for typical preclotting during surgery; however, such coating renders the tube stiffer and more difficult to handle during implantation. Also such a coated product is considerably more expensive than a simple woven graft.
The above description of prior art grafts shows some of the numerous parameters considered in the selection of vascular grafts. Additional factors include tissue compatibility, nonthrombogenicity of the surface, deterioration of the graft with time, resistance to infection, and resistance to kinking at the joints of the patient.
Typically in the manufacture of both knitted and woven grafts the tubular body is crimped to form circumferential corrugations or ribs that provide strength and resilience against kinking and collapsing of the tube and narrowing of the lumen from bending or twisting. Known woven grafts use a polyester such as Dacron.RTM. (polyethylene terephythalate) yarn, Type 56 made by Dupont for approximately 30 years and designated 40 denier/27 or 70 denier/34, the 40d/27 representing 40 grams of weight per 9000 meters of yarn which yarn comprises 27 filaments, or 1.48 denier per filament. Dacron.RTM. is a registered trademark for E. I. Dupont for polyester yarn. The selection of 40d/27 yarn in single or double ply as the standard of the industry has been dictated by what was available on the market and what has been approved by the F.D.A. Prior patents referred to above describe more fully this standard yarn, which is also texturized in a standard way, i.e. twisting the fibers at about 50,000 rpm, under 8- 15 grams of tension at about 450.degree. F.
The weaving of arterial grafts is done on known weaving apparatus with a matrix of weft or fill threads into which are woven a pattern of warp threads, some of which are twill or velour weave to later produce the velour loops on the inner and/or outer surfaces. As is known, these twill threads for the velour loops are preshrunk, so that upon the shrinking of the completed graft tube made of otherwise unshrunk yarn, all threads will shrink except the velour ones which will extend outward from the surface as loops to subsequently receive the tissue ingrowth.
In view of the above-described function parameters and the differences between the various prior art grafts, compromises in characteristics are required with each selection. More specifically, if one wishes softness and pliability and porosity, the choice must be a knit graft with the required preclotting.